1. Field of the Invention
The invention is directed to segmented retractable packing rings for use with a rotating shaft of a steam turbine.
2. Description of the Prior Art
Reductions in steam turbine efficiency and performance are caused by many factors, among which is steam leakage through the packing rings.
The typical packing ring is formed into arcuate toothed segments held by a hook fit in diaphragm or holder segments so as to circumscribe the shaft and to be movable toward and away therefrom by spring action and/or steam pressure.
The springs hold the segments in minimum clearance position. There is, on average, 0.025" of clearance between the rotating shaft of the turbine and the packing rings. Most turbine manufacturers design packing rings with the springs forcing the packing ring segments toward the shaft to a close clearance position.
The packing is designed so that as steam flows past each tooth, there is a pressure drop. This pressure drop is relatively linear in nature and operates as an opening force.
As closing forces, upstream steam pressure and the spring force act on the back side of the packing ring and downstream pressure past the steam joint face acts on the back side of the packing ring.
The forces, acting in the closing direction, will always be greater than those in the opening direction.
The springs hold the segments against the hook fit, creating a close clearance between the packing segment and the rotating shaft. As steam pressure builds up on the upstream side of the packing it causes the packing segment to move downstream against the hook fit shoulder, forming a sealing surface. The force holding the packing ring radially toward the shaft is a combination of the spring force and the force of the steam leaking into the area on the backside of the packing ring. As the steam flow increases, the ability of the spring to allow the packing segment to move away from the shaft is significantly reduced. High flows eliminate radial movement of the packing segment, due to the steam force and the coefficient of friction that exists between the packing and the holder at the steam seal face. Under this condition, the shaft packing becomes essentially rigid.
With the packing rings of the prior art, the inner or tooling diameter or profile of the packing ring segments does not match the inner or tooling diameter or profile of the diaphragm or holder for the ring segment, the diameters or profiles differing by as much as 1/4 inch, so that objectionable rocking of the ring in thee holder occurs in the retracted or opened position.
Thus, the steam in the turbine does not act uniformly on the front and back of the packing, wherefore the packing does not close in a uniform manner.
In addition, this makes it virtually impossible to accurately calculate the forces required for closing the packing, wherefore it is impossible to determine the exact time of closure.